测试三种常用的字符串判空串方法所耗的时间，出什么问题了？

using System;
using System.Threading;
public class strLengthTime
{
public static void Main()
{
string strTest="";
string strSub;
for(int i =0 ;i< 1000;i++)
{
strSub=i.ToString();
strTest = strTest + strSub;
}
//System.Console.WriteLine(strTest);
DateTime startTime;
DateTime endTime;
TimeSpan timeCost;
startTime =DateTime.Now;
if(strTest.Length==0)
{
System.Console.WriteLine("Not A Null String!");
}
else
{
endTime=DateTime.Now;
timeCost=endTime - startTime;
System.Console.WriteLine("strTest.Length Method Costs:"+timeCost.ToString());
}
Thread.Sleep(1000);
startTime = DateTime.Now;
if(strTest==String.Empty)
{
System.Console.WriteLine("Not A Null String!");
}
else
{
endTime=DateTime.Now;
timeCost= endTime - startTime;
System.Console.WriteLine("String.Empty Method Costs:"+timeCost.ToString());
}
Thread.Sleep(1000);
startTime = DateTime.Now;
if(strTest=="")
{
System.Console.WriteLine("Not A Null String!");
}
else
{
endTime=DateTime.Now;
timeCost= endTime - startTime;
System.Console.WriteLine("The Third Method Costs:"+timeCost.ToString());
}
}
}

输出的结果都是00：00：00，出什么问题了？